NOTE: THE FOLLOWING FIXES ARE ONLY FOR HORIZONTAL GAPS, THEY DO NOT WORK FOR VERTICAL W10 MAIL GAPS.

Okay so Stefano created an initial thread querying this and everyone seemed to have varied results on what fixes it - it seems to depend on the creative and table structure. I thought it would be best to clearly post all the fixes that we've found to work, Kudos to everyone in the thread that helped us get to this stage:

So you have two sliced images, one ontop of another but there's a 1px gap between them which displays as the same color as your body background, right?

A suggested hack was to change the bgcolor for the "line" by wrapping a table around your images and setting a new BG color on the table - but this isn't a fix just a temporary solution and it won't work with every creative.

Next a user suggested that the wrapping table was too long, for me I found this to be an issue as once I broke my wrapping table into two tables the line disappeared on one of my two broken areas.

Next I tried putting them back into the wrapping table and added an additional spacer table above the broken area. Now to my surprise the "line" moved down into the next section(one which I had repeated for testing), so I turned a single spacer in two two separate spacers and this pushed the line below my images and thus it wasn't an issue for my creative anymore.

So I had 2 broken sections, and I found two fixes for the section on the right hand side - but regardless to how many spacers I added above the left hand side section, the line just wouldn't move. I tried repeating the entire section and that did move the line down to the repeated section but once I removed it - it moved back up.

Finally I found a little bit of code from a user and they stated that it helped them remove the spaces below VMLs - this fixed my issue on the left hand side section: <div align="right" style="font-size: 0; line-height: 0;"> </div>

So I wrapped my image and anchor tag with this DIV (only the top of the 2 sliced images) and this rectified the issue!

In short:
0. If changing the bgcolor won't help....
1. Try adding an extra/additional table/s above the broken section
2. Try Duplicating the broken section
3. Try wrapping the anchor and image tag in the DIV mentioned above.

I hope this was of some help, it was driving me insane! Here's a link to the original thread:
https://litmus.com/community/discussions/4990-outlook-2016-1px-horizontal-lines-showing-up-in-the-body

And special thanks to:
Stefano Bagnara
Kathiravan Kumarasamy
Nicole Merlin